1. Field of the Invention
This invention relates to a molecular beam epitaxy (MBE) method for the epitaxial growth of Group III nitride semiconductor materials such as, for example, GaN or other members of the (Ga, Al, In)N material family.
2. Description of the Related Art
The epitaxial growth of Group III nitride semiconductor materials on a substrate can be effected by molecular beam epitaxy (MBE) or by chemical vapour deposition (CVD) which is sometimes known as Vapour Phase Epitaxy (VPE).
CVD (or VPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
In contrast to CVD, MBE in carried out in a high vacuum environment. In the case of MBE as applied to the GaN system, an ultra-high vacuum (UHV) environment, typically around 1xc3x9710xe2x88x923 Pa, is used. Ammonia or another nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and a species providing gallium and, possibly, indium and/or aluminium are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The ammonia and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
At present, the majority of growth of high quality GaN layers is carried out using the metal-organic chemical vapour deposition (MOCVD) process. The MOCVD process allows good control of the growth of the nucleation layer and of the annealing of the nucleation layer. Furthermore, the MOCVD process allows growth to occur at a V/III ratio well in excess of 1000:1. The V/III ratio is the molar ratio of the group V element to the Group III element during the growth process. A high V/III ratio is preferable, since this allows a higher substrate temperature to be used which in turn leads to a higher quality GaN layer.
At present, growing high quality GaN layers by MBE is more difficult than growing such layers by MOCVD. The principal difficulty is in supplying sufficient nitrogen during the growth process; it is difficult to obtain a V/III ratio of 10:1 or greater. The two commonly used sources of nitrogen In the MBE growth of nitride layers are plasma excited molecular nitrogen or ammonia.
GaN has a lattice constant of around 0.45 nm. There is a lack of suitable substrates that, are lattice-matched to GaN, so GaN is generally grown onto either a sapphire substrate or a silicon carbide substrate. There is a large mis-match between the lattice constant of GaN and the lattice constant of sapphire or silicon carbide, and there is also a considerable difference in thermal properties, such as the thermal expansion coefficient, between the GaN layer and the substrate. It is therefore necessary to provide a thin initial nucleation layer on the substrate in order to grow a high quality GaN layer on sapphire or silicon carbide.
I. Akasaki and I. Amano report, in xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d Vol. 36, pp5393-5408 (1997), that a thin AlN layer, deposited at a low growth temperature, can be used as a nucleation layer to promote the growth of a GaN layer by metal organic chemical vapour deposition (MOCVD) process on a sapphire or silicon carbide substrate. U.S. Pat. No. 5,290,393 discloses the use of a GaN nucleation layer, again deposited at a low growth temperature, for promoting the growth of a GaN layer using MOCVD.
U.S. Pat. No. 5,385,862 discloses a further method of growing a single crystal GaN film on a sapphire substrate using MBE. In this method, a nucleation layer is grown on the substrate at a growth temperature of 400xc2x0 C. or lower. Furthermore, the V/III ratio of this method is very small, being less than 5:1, so that the subsequent GaN layer is restricted to a growth temperature of lower than 900xc2x0 C. GaN layers grown by this method have electron mobilities at room temperature of less than 100 cm2Vxe2x88x921sxe2x88x921.
Further prior art methods of growing GaN on a sapphire or silicon carbide substrate are reported by Z. Yang et al in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 67, pp1686-1688 (1995), and by N. Grandjean et al in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 71, p240-242 (1997). In both of these methods a GaN nucleation layer is initially grown on the substrate, after which the GaN layer is grown.
Although the provision of a nucleation layer does reduce the effect of the lattice and thermal mis-match between a GaN layer and a sapphire or silicon carbide substrate, the effects of the lattice and thermal mis-match are not eliminated completely. Moreover it is difficult and time-consuming to optimise the nucleation layer so as to obtain the highest possible quality GaN, and the step of growing the nucleation layer adds to the complexity of the growth process. It is accordingly desirable to use a GaN substrate for the growth of an epitaxial GaN layer.
A GaN substrate for use in the epitaxial growth of GaN can have two possible formsxe2x80x94a GaN substrate can be a xe2x80x9cfree-standingxe2x80x9d substrate or a xe2x80x9ctemplatexe2x80x9d substrate. A free-standing GaN substrate consists solely of GaN, and is formed by, for example, a GaN crystal. A template GaN substrate consists of a thick epitaxial layer of GaN grown on a base substrate of, for example, sapphire or silicon carbide. The thick epitaxial layer is grown on the base substrate by any suitable technique, such as metal-organic vapour phase epitaxy (MOVPE) or hydride vapour phase epitaxy (HVPE). Compared with the nucleation layers mentioned above, the epitaxial layer of a GaN template substrate is much thicker than a nucleation layer, for example having a thickness in the range 5 xcexcm-100 xcexcm.
M. Kamp et al report, in xe2x80x9cMat Res Soc Procxe2x80x9d, Vol. 449, p161 (1997), the growth of a GaN layer by MBE on a free-standing GaN substrate. They obtain good quality GaN, having a photoluminescence (PL) linewidth with a FWHM of 0.5 meV and a dislocation density in the range 102 to 103 cmxe2x88x922. However, Kamp et al achieve a growth temperature of only 750xc2x0 C.
WO 97/13891 discloses a method of epitaxial growth of a nitride semiconductor layer (GaN or Ga(Al,In)N) on a single crystal GaN or Ga(Al,In)N. This document is primarily directed to the way in which the substrate is produced, and teaches disposing a solution of Ga or Ga,Al,In in a heated nitrogen atmosphere so as to grow a bulk crystal of GaN or Ga(Al,In)N.
Once the bulk crystal has been grown, it is used as the substrate in an epitaxial growth process. The document speculates that it would be possible to grow an epitaxial layer on the substrate by MBE in the temperature range 500-900xc2x0 C. however, the document contains no teaching as to how an MBE growth temperature for GaN of 900xc2x0 C. could be achieved.
The growth of GaN on a GaN template substrate has been reported by, for example, W. C. Hughes et al in xe2x80x9cJ. Vac. Sci. Technol Bxe2x80x9d Vol. 13, p1571 (1995). In this report, GaN is grown by MBE with plasma excited molecular nitrogen used as the source of nitrogen for the MBE growth process. Other reports of the MBE growth of GaN on a GaN template substrate have been made by E. J., Tarsa et al in xe2x80x9cJournal of Applied-Physicsxe2x80x9d, Vol. 82, p5472 (1997); by H. Sakai et al in xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d, Vol. 34, L1429 (1995); by M. A. Sanchez-Garcia at al in xe2x80x9cPhys. Stat. Sol.(a)xe2x80x9d, Vol. 176, p447 (1999); by S. Kurai et al in xe2x80x9cPhys. Stat. Sol.(a)xe2x80x9d, Vol. 176, p459 (1999): and by A. Rinta-Moykky et al in xe2x80x9cPhys. Stat. Sol.(a)xe2x80x9d, Vol. 176, p465 (1999). In each of these reports, plasma excited molecular nitrogen was used as the source of nitrogen for the MBE growth process.
A first aspect of the present invention provides a method of growing a nitride semiconductor layer by molecular beam epitaxy (MBE) comprising the steps of: heating A GaN substrate disposed in a growth chamber to a substrate temperature of at least 850xc2x0 C.; and growing a nitride semiconductor layer on the GaN substrate by molecular beam epitaxy at a substrate temperature of at least 850xc2x0 C., ammonia gas being supplied to the growth chamber during the growth of the nitride semiconductor layer; wherein the method comprises the further step of commencing the supply ammonia gas to the growth chamber during step (a), before the substrate temperature reaches 800xc2x0 C. The present invention thus provides a method of growing a high quality layer of a nitride semiconductor by MBE. Commencing the supply of ammonia gas to the substrate during the step of heating the substrate, before the substrate temperature reached 800xc2x0 C., prevents thermal decomposition of the substrate (which would otherwise occur at substrate temperatures above 800xc2x0 C.). The substrate temperature during the growth of the nitride semiconductor layer may be in the range 850xc2x0 C. to 1050xc2x0 C.
The substrate temperature during the growth of the nitride semiconductor layer may be in the range 850xc2x0 C. to 1050xc2x0 C.
The method may comprise the further step of reducing the substrate temperature to a temperature below 800xc2x0 C. after the nitride semiconductor layer has been grown while maintaining the supply of ammonia gas to the growth chamber.
The method may comprise the further step of maintaining the substrate at a substrate temperature greater than 850xc2x0 C. for a predetermined time before growing the nitride semiconductor layer, ammonia gas being supplied to the growth chamber during the predetermined time. This allows the substrate to be out-gassed, thereby removing impurities from the substrate before the start of the growth process.
The substrate temperature may be maintained in the range 850xc2x0 C. to 1050xc2x0 C. during the predetermined time.
The predetermined time may be 30 minutes or less.
Ammonia gas may be supplied to the growth chamber during the entire duration of step (a).
The nitride semiconductor layer may be a GaN layer. The substrate may be a freestanding GaN substrate, or it may alternatively be a GaN template substrate. The use of a GaN substrate eliminates the lattice and thermal mis-match that occurs when, for example, a GaN layer is grown on a sapphire or silicon carbide substrate.
The method may comprise the further step of growing at least one (Al,Ga,In)N semiconductor layer on the nitride semiconductor layer. Thus, the present invention enables a high-quality (Al,Ga,In)N electronic or opto-electronic device to be manufactured.
A second aspect of the invention provides a nitride semiconductor layer grown by a method as defined above.
A third aspect of the present invention defines a semiconductor device comprising a nitride semiconductor layer as defined above.
Preferred embodiments of the present invention will now be described by way of an illustrative example with reference to the accompanying figures which: